The present invention relates to electrical cables, and particularly to woven electrical cables used for probing applications which require multiple probes, for example, measuring instruments such as analog or digital oscilloscopes or logic analyzers.
Electrical cables are used to transfer electrical signals from one point to another. Three important concerns of electrical cable design are maintaining the integrity of the signals during transfer, maintaining the integrity of the system under test, and withstanding physical manipulation without being bulky, heavy, or difficult to use. One economical method of providing protection from physical impacts and abrasions which is well-known is to encase the electrical cable in a strong, durable, woven cable.
A type of cabling method used in the prior art required the use of a coaxial cable with a resistive center conductor for each signal to be probed. For example, a logic analyzer having 16 probes would require 16 coaxial cables tied together from the logic analyzer to the system under test. This tended to greatly improve the performance of the probing system, since the shielding was greatly improved, bandwidth was increased, and crosstalk was reduced. However, the result was an extremely bulky, costly, and heavy cable, making it difficult for the user to operate with a typical system under test.
A second prior art signal transference system used with a measuring instrument having multiple probes is shown in FIG. 1. This system used resistor-capacitor attenuation circuits for each probe and had a pod 10 containing active electronics located in close proximity to the probe tips 20. The system used a twisted pair cable 90 from the pod 10 to the measuring instrument and used a relatively short single wire 100 (approximately ten inches long) from the pod 10 to the probe tip 20 for each probe. Each probe tip 20 was connectable to the system under test. The pod 10 contained drivers to actively transmit the signals to the measuring instruments on the twisted pair cable 90. The twisted pair cable 90 provided some shielding and avoidance of crosstalk once the signals from the system under test reached the pod 10. Prior to the signals reaching the pod 10, however, crosstalk developed in the wire 100 because the wire 100 and the ground wire 110 formed a lossless transmission line with very little shielding. Also, the probes had a higher input capacitance because the resistor-capacitor attenuation networks were located in the pod and not at the probe tips. This limits the effective bandwidth of the probes because of the resonant circuit formed by the single wire inductance and its stray capacitance.